The invention relates to a coaxial cable and more particularly, to corrosion-protected trunk and distribution cable and drop cable for the transmission of RF signals.
RF signals such as cable television signals, cellular telephone signals, and even internet and other data signals, are often transmitted through coaxial cable to a subscriber. In particular, the RF signals are typically transmitted over long distances a using trunk and distribution cable and drop cables are used as the final link in bringing the signals from the trunk and distribution cable to the subscriber. Trunk and distribution cable and drop cable both generally include a center conductor, a dielectric layer, an outer conductor and often a protective jacket to prevent moisture from entering the cable.
One problem associated with these coaxial cables is that moisture present in the cable can corrode the conductors thus negatively affecting the electrical and mechanical properties of the cable. In particular, during installation of the cable, moisture can enter the cable at the connectors. This moisture can also travel within the cable through the dielectric layer or along interfaces in the cable, e.g., between the dielectric layer and the outer conductor.
Several methods have been proposed to prevent moisture from entering the cable and being transported through the cable. For example, hydrophobic, adhesive compositions have been applied at interfaces in the cable to prevent moisture from moving along these interfaces. Flooding or water-blocking compositions have also been used at other locations in the cable to limit water transport in the cable. In addition, hydrophilic, moisture-absorbent materials have been used in cables to act as water-blocking materials. These hydrophilic materials not only water-block the cable but also remove moisture that is present in the cable.
Although these materials can provide adequate protection from moisture and can limit corrosion of the conductors in the cable, these materials have a tacky or greasy feel and thus are undesirable during the installation and connectorization of the cable, particularly when located on the outer conductor of the cable. As a result, these materials generally must be removed or otherwise addressed during installation and connectorization of the cable. Therefore, there is a need to provide a corrosion-inhibiting coating for cable that does not possess a tacky or greasy feel and thus that does not interfere with installation and connectorization of the cable.
The present invention provides a corrosion-protected cable that includes a corrosion-inhibiting coating that limits and even prevents the corrosion of the conductors, and particularly the outer conductor, of the cable. In addition, the present invention includes a corrosion-inhibiting composition and a method of applying the corrosion-inhibiting composition to the outer conductor of a cable. The composition when heated forms a corrosion-inhibiting coating on the surface of the outer conductor that is not tacky or greasy and thus is desirable in the art.
According to one embodiment of the invention, the present invention includes a coaxial cable, comprising an elongate center conductor, a dielectric layer surrounding the center conductor, an outer conductor surrounding the dielectric layer, a corrosion-inhibiting coating on at least an outer portion of the outer conductor, and preferably a polymer jacket around the outer conductor. The center conductor is preferably formed of a material selected from the group consisting of copper, a copper alloy, a copper-clad metal, and a copper alloy-clad metal. The dielectric layer preferably comprises a foamed polymeric material. The cable can further include a corrosion-inhibiting layer between the center conductor and the dielectric layer comprising a benzotriazole compound (e.g. BTA) and a polymeric compound (e.g. a foamed, low-density polyethylene). The outer conductor is preferably formed of aluminum or an aluminum alloy but can be copper or another conductive material. For example, the outer conductor can include a bonded aluminum-polymer-aluminum laminate tape extending longitudinally of the cable preferably having overlapping longitudinal edges and the corrosion-inhibiting composition can be applied to an outer surface of said laminate tape. The outer conductor can further include a plurality of braided or helically arranged wires coated with the corrosion-inhibiting composition. Alternatively, the outer conductor can include a longitudinally-welded sheath and the corrosion-inhibiting composition can be applied to an outer surface of the sheath. The corrosion-inhibiting coating comprises a corrosion-inhibiting compound selected from the group consisting of petroleum sulfonates, benzotriazoles, alkylbenzotriazoles, benzimidazoles, guanadino benzimidazoles, phenyl benzimidazoles, tolyltriazoles, metcaptotriazoles, mercaptobenzotriazoles, and salts thereof. In addition, the corrosion-inhibiting coating can include a residual amount of an oil dispersant and/or a residual amount of a stabilizer.
In accordance with the invention, the corrosion-inhibiting composition includes a water-insoluble corrosion-inhibiting compound dispersed in an oil, and a stabilizer selected from the group consisting of propylene based glycol ethers, propylene based glycol ether acetates, ethylene based glycol ethers and ethylene based glycol ether acetates. The stabilizer is preferably selected from the group consisting of dipropylene glycol methyl ether acetate, propylene glycol methyl ether, dipropylene glycol methyl ether, tripropylene glycol methyl ether, propylene glycol t-butyl ether, propylene glycol methyl ether acetate ethylene glycol methyl ether, ethylene glycol ethyl ether, ethylene glycol butyl ether , diethylene glycol methyl ether, diethylene glycol ethyl ether, diethylene glycol butyl ether, ethylene glycol ethyl ether acetate, ethylene glycol butyl ether acetate, diethylene glycol ethyl ether acetate, diethylene glycol butyl ether acetate, and mixtures thereof, and is more preferably a dipropylene glycol ether acetate (e.g. dipropylene glycol methyl ether acetate). The corrosion-inhibiting compound is selected from the group consisting of petroleum sulfonates, benzotriazoles, alkylbenzotriazoles, benzimidazoles, guanadino benzimidazoles, phenyl benzimidazoles, tolyltriazoles, metcaptotriazoles, mercaptobenzotriazoles, and salts thereof, and is preferably a petroleum sulfonate salt. The petroleum sulfonate salt is selected from the group consisting of calcium, barium, magnesium, sodium, potassium and ammonium salts, and mixtures thereof, and is preferably a calcium salt having an activity of greater than 0 to about 25% based on the calcium salt. The calcium salt optionally further includes a salt selected from the group consisting of barium and sodium salts. The oil is preferably a paraffinic oil such as a mineral oil that preferably has a molecular weight of less than about 600. The corrosion-inhibiting composition preferably includes the corrosion-inhibiting compound in an amount of from about 5 to about 40% by weight, the oil in an amount of from about 50 to about 90% by weight, and the stabilizer in an amount of from about 1 to about 10% by weight. More preferably, the corrosion-inhibiting composition includes the corrosion-inhibiting compound in an amount of from about 15 to about 30% by weight, the oil in an amount of from about 60 to about 80% by weight, and the stabilizer in an amount of from about 3 to about 8% by weight. The corrosion-inhibiting composition preferably also has a viscosity of from about 50 to about 450 SSU at 100xc2x0 F. The corrosion-inhibiting composition can be heated to form the corrosion-inhibiting coating of the invention that is present on at least a portion of the outer surface of the outer conductor.
The present invention further includes a method of making a coaxial cable, comprising the steps of advancing a center conductor along a predetermined path of travel, applying a dielectric layer around the center conductor, applying an outer conductor around the dielectric layer, and applying the corrosion-inhibiting composition to the outer conductor. The cable can then be heated to produce the corrosion-inhibiting coating, e.g., by applying a polymer melt around the outer conductor to form a protective jacket. The outer conductor can be formed by directing an aluminum-polymer-aluminum laminate tape around the dielectric layer and overlapping longitudinal edges of the laminate tape to form the outer conductor. The outer conductor can also include a plurality of wires formed into a braid or helically arranged around the laminate tape and the corrosion-inhibiting composition applied to the wires by wiping the wires with the corrosion-inhibiting composition. The corrosion-inhibiting composition can also be applied to the outer conductor by wiping the outer surface of the laminate tape with the corrosion-inhibiting composition or immersing the cable in the corrosion-inhibiting composition prior to forming the braid or helically arranging the wires. Alternatively, the corrosion-inhibiting composition can be applied to the outer conductor by wiping the outer surface of the outer conductor with the corrosion-inhibiting composition or immersing the cable in the corrosion-inhibiting composition after forming the braid or helically arranging the wires. The outer conductor can also be formed by directing an aluminum strip around the dielectric layer and longitudinally-welding abutting edges of the metal strip, and the corrosion-inhibiting composition applied to the outer conductor by wiping the outer surface of the outer conductor with the corrosion-inhibiting composition or by immersing the cable in the corrosion-inhibiting composition.